Detecting particles in a gas flow

ABSTRACT

In a method for detecting particles in a gas flow, a probe 8 is charged triboelectrically by particles 10 in the flow and the quantities of electrical charges transferred to the probe are evaluated to provide an indication of the particle flow in the gas flow. In order to reduce the effect of variations in &#34;gas flow related variables&#34; other than those relating to particle flow, an alternating component in the signal caused by the triboelectrical charging of the probe is monitored.

This invention relates to an apparatus and method for detectingparticles in a gas flow. The invention is particularly concerned with anarrangement in which particles are detected as a result of thoseparticles triboelectrically charging a probe in a duct along which a gasflows.

WO 86/02454 describes an apparatus which uses triboelectric effects. Ametal probe projects into a duct and particles carried past the probe inthe gas flow impart triboelectric charges to the probe. Those chargesare conducted to ground through an electric circuit, resulting in anelectric current. The current is converted to a unipolar voltage andamplified to provide an indication of the mass of the particles.

In WO 86/02454 an auto-zero circuit is preferably provided to rezero thecircuit periodically (about once per minute) in order to prevent theoutput of the circuit varying with ambient temperature as a result ofchanging electrical characteristics of the current to voltage conversioncircuit. As an alternative to the auto-zero circuit a system which usesonly the a.c. component of the signal can be used apparently for thesame purpose. In commercial versions of the apparatus described in WO86/02454, the auto-zero circuit is employed.

We have found that apparatus of the kind described in WO 86/02454 isunable to give a reliable indication of the mass flow rate of particlesin the gas flow and that the magnitude of the current generated by thecharges is influenced by factors other than the mass of the particles.

It is an object of the invention to provide a method and apparatus fordetecting particles in a gas flow which relies upon particles in theflow triboelectrically charging a probe and which is less affected bychanges in other variables relating to the gas flow.

According to the invention there is provided a method for detectingparticles in a gas flow in which a probe is charged triboelectrically byparticles in the flow and the quantities of electrical chargestransferred to the probe are evaluated to provide an indication of theparticle flow in the gas flow, wherein, in order to reduce the effect ofvariations in "gas flow related variables" other than those relating toparticle flow, an alternating component in the signal caused by thetriboelectrical charging of the probe is monitored. The term "gas flowrelated variables" refers to variables that are related to theenvironment in the region of the gas flow and at the probe, to the gasflow itself and/or to the particles in the gas flow. Examples of gasflow related variables other than those relating to particle flow aretherefore: humidity in the region of the gas flow, temperature of thegas flow, thickness of a particulate layer deposited on the probe, andthe electrical charge of particles in the gas flow. Examples of gas flowrelated variables that relate to particle flow are the mass flow rate ofthe particles, and the velocity and size of the particles.

We have discovered that by looking at the alternating component in thesignal from the triboelectrically charged probe rather than looking atthe absolute value of that signal, it is possible to obtain an outputsignal that is much less affected by gas flow related variables, otherthan those relating to particle flow, than in the case of, for example,the apparatus described in WO 86/02454. Although it might appear to bedisadvantageous to evaluate the much smaller amplitude alternatingsignal component involved, we have found that such disadvantages aremore than compensated for by the much improved correlation between, forexample, the mass flow rate of particles in the flow and the alternatingcomponent in the signal from the probe. More particularly, we have foundthat whereas variations in gas flow related variables other than thoserelating to particle flow are liable to affect the absolute value of asignal from a probe charged triboelectrically, they are not likely toaffect the magnitude of the alternating component of the signal to thesame extent. This hitherto undiscovered phenomenon is not fullyunderstood but it is believed that factors such as humidity, electricalcharges already on the particles and a build-up of particles on theprobe all affect the absolute value of the current without affecting thealternating component of the current as much.

Preferably the alternating component of the signal from the probe isfiltered to limit the frequency to below about 2 Hz, preferably about1.5 Hz. By eliminating higher frequencies the risk of spurious signalsderived from mechanical vibration of the probe is substantially reducedsince the resonant frequency of such vibration is likely to besubstantially higher than 2 Hz.

Preferably the alternating component of the signal from the probe isfiltered to limit the frequency of the signal to above about 0.1 Hz,preferably about 0.15 Hz. By eliminating lower frequencies the risk ofspurious signals derived from transient temperature-generated voltagesis substantially reduced.

Preferably the alternating component of the signal from the probe isamplified in a plurality of stages. In that case low frequencies, whichmay be those below 0.15 Hz, are preferably attenuated at the first stageof amplification.

In the most common case, the size and composition of the particles willnot vary and the flow will be monitored in order to detect variations inthe mass flow rate. Given that the size of the particles and theircomposition does not vary, the measurement of mass flow rate canalternatively be regarded as a measurement of the flow rate in terms ofthe number of particles per unit time.

Usually it will be desired to provide a quantitative indication of themass flow rate but for some applications it may be adequate simply toprovide an indication of whether or not the mass flow rate measured isabove or below some threshold level. An alarm may be sounded if the massflow rate is above the threshold level.

Usually, the gas flow is along a duct and the probe is in the duct.

The present invention also provides the use, in an apparatus fordetecting particles in a gas flow, which apparatus includes a probe forinstallation in the gas flow, of an electric circuit for generating anelectric signal from electrical charges transferred to the probe as aresult of triboelectric charging thereof by particles in the flow, andevaluating means for providing an output signal in dependence upon analternating component of the electric signal generated in the circuit,as a means for reducing the effect of variations in "gas flow relatedvariables" other than those relating to particle flow.

The invention can be used for continuous measurement of mass flow rateof particles in applications where particles are suspended in a gasflow. Examples of applications are: continuous measurement of unwantedparticulate emissions from stacks and dust arrestment plants andcontinuous measurement of particle mass flow rates in systems whereparticles are suspended in a gas stream, for example in a pneumaticconveying system.

Dust flow monitoring apparatus in accordance with the invention will nowbe described by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 is a block-diagram representation of the electrical system of thedust flow monitoring apparatus,

FIG. 2 is a schematic sectional view of a sensing head mounted on astack along which dust-carrying air is passing,

FIG. 3 is a circuit-diagram representation of an input amplifier forminga part of the electrical system,

FIG. 4 is a circuit-diagram representation of a coupling network and asecond stage amplifier forming another part of the electrical system,

FIG. 5a is a graph of the current from the probe plotted against time ina case where the particle flow remains constant but other variables inthe duct alter,

FIGS. 5b to 5d are graphs of signal amplitude against time for differentsignals that can be derived from the current signal of FIG. 5a, and

FIG. 6 is a graph showing the a.c. and d.c. current outputs from a probemonitoring an air flow to which coal particles are added at a constantrate.

Referring to FIG. 1 of the accompanying drawings, the electrical systemof the dust flow monitoring apparatus includes a sensor 1, an inputamplifier 2, a first coupling network 3, a second stage amplifier 4, again-change logic circuit 5, a difference amplifier 100, a low-passfilter and fine-gain amplifier 101, a second coupling network 102, anactive rectifier 103, an averaging filter and output amplifier 104, avoltage-to-current converter 105, an alarm logic and controller 106, anda coarse gain switching controller 107.

The sensor 1, the input amplifier 2, the first coupling network 5 andthe second stage amplifier 4 are connected in cascade. The gain-changelogic circuit 5 has a first connection connected to the input amplifier2 and to the second stage amplifier 4, and a second connection connectedonly to the input amplifier 2. The coarse gain switching controller 107is connected to the gain-change logic circuit 5.

The difference amplifier 100, the low-pass filter and fine-gainamplifier 101, the second coupling network 102, the active rectifier 103and the averaging filter and output amplifier 104 are connected incascade. The voltage-to-current converter 105 and the alarm logic andcontroller 106 are connected to the averaging filter and outputamplifier 104.

Referring now also to FIG. 2 the sensor 1, the input amplifier 2, thefirst coupling network 3, the second stage amplifier 4 and thegain-change logic circuit 5 form a sensing head 6 which, in use, ismounted on a stack 7, the dust flow along which is being monitored. Theremainder of the electrical system is "control room" equipment and islocated at a position remote from the sensing head. The second stageamplifier 4 of the sensing head 6 is connected to the differenceamplifier 100 of the "control room" equipment by way of a connectionmeans 108 which might include a length of cable. The gain-change logiccircuit 5 is connected to the coarse gain switching controller 107 byway of the connection means 108.

The sensor 1 includes a conducting rod 8 forming a probe which projectsinto the stack. The conducting rod 8 is covered by an insulating member9 which may be of a ceramic or PTFE material and the insulating member 9extends some of the way along the conducting rod 8 towards its free endbut stops short of the end.

The input amplifier 2 is a shunt-feedback current amplifier whichconverts its input current, which is the current supplied by theconducting rod, into an output voltage. The amplifier 2 is d.c. coupledand has switchable components providing selectable transimpedance gainsof between 0.1 millivolts per picoampere and 40 millivolts perpicoampere in four steps. The four steps provide transimpedance gains of0.1 millivolts per picoampere, 0.4 millivolts per picoampere, 10millivolts per picoampere and 40 millivolts per picoampere,respectively. The input amplifier 2 also includes capacitance resistancefeedback networks which set the upper frequency response at about 1.5Hz.

The output signal from the input amplifier 2 passes to the firstcoupling network 3 which includes a series capacitor 3100 and shuntresistor 3200. The series capacitor 3100 blocks the d.c. and very lowfrequency signals from the input amplifier 2, the capacitor 3100 andresistor 3200 being selected to set the lower frequency response of thesignal path at 0.15 Hz.

The signal passing through the capacitor 3100 next goes to the secondstage amplifier 4 which is a d.c. coupled voltage amplifier havingswitchable gain-setting means for setting its gain to 2 or to 5.

The gain switching arrangements of the amplifiers 2 and 4 are so linkedas to provide overall transimpedance gains of 0.2 millivolts perpicoampere, 2 millivolts per picoampere, 20 millivolts per picoampereand 200 millivolts per picoampere, respectively.

The settings of the transimpedance-gain of the input amplifier 2 and thevoltage gain of the second stage amplifier 4 are effected by thegain-change logic circuit 5 operating under the control of the coarsegain switching controller 107 which is controlled manually.

The selected maximum transimpedance gain of the input amplifier 2provides a good signal-to-noise ratio for the system by ensuring that asignificant proportion of the required system gain is provided at theinput stage without raising significant difficulties oftemperature-generated output voltage. The potential difficulty oftemperature-generated output voltage is also met by limiting the lowerfrequency of the transmission path to 0.15 Hz by means of the firstcoupling network 3. The selection of the upper frequency limit as 1.5 Hzis effective to counter the effects of mechanical vibration and noisewhile providing a bandwidth adequate for providing accurate informationon the flow rate of dust particles impinging on the sensor 1. That is,the system bandwidth is carefully selected in order to counter a rangeof system effects which generate signals likely to cause errors in thefinal result. Other turn-over frequencies may be used at the cost ofreduced effectiveness of the system in discriminating against unwantedeffects.

The bandwidth-limited output signal from the second stage amplifier 4passes to the difference amplifier 100 where it is subjected toadditional bandwidth-shaping by means of capacitor-resistor networks inorder to improve the high-frequency roll off above 1.5 Hz. Thedifference amplifier 100 is a differential amplifier and has a highcommon-mode rejection ratio. The capacitor-resistor networks include aparallel capacitor-resistor network shunting the non-inverting inputterminal of the amplifier and another parallel capacitor-resistornetwork connected between the inverting input terminal of the amplifierand its output terminal.

The signal next passes to the low-pass filter and fine-gain amplifier101 where further low-pass characteristic shaping is applied by means ofcapacitance-resistance networks which provide a 12 bB/octave roll-offabove 1.5 Hz. The low-pass filter and fine-gain amplifier also providescontinuously adjustable voltage gain of between 1 and 10. Thecapacitor-resistor networks include two resistors connected in serieswith each other and with the non-inverting terminal of the amplifier, acapacitor connected between the junction of the two resistors and theinverting terminal of the amplifier, and a further capacitor connectedbetween the non-inverting terminal of the amplifier and a groundingpoint of the system.

The second coupling network 102 receives the signal from the low-passfilter and fine-gain control amplifier 101. The second coupling network102 has a series capacitor 1021 and a shunt resistor 1022 and serves toblock temperature-generated signals and time-dependent d.c. signalsintroduced after the first coupling network 3.

The signals passing through the coupling network 102 go to the activerectifier 103 which also provides a voltage gain of 2. The signals thenpass to the averaging filter and output amplifier 104 which provides along-term average of the signals, reducing the random signal variationswhich particle flow provides. The averaging filter and output amplifieralso provides a voltage gain of 5.

The averaging filter and amplifier 104 provides signals for avoltage-to-current converter 105 for driving a pen-recorder or the like.The voltage-to-current converter is capable of providing a 4 to 20 mAoutput current swing for an input voltage swing of 0 to 10 volts. Theaveraging filter and amplifier 104 also provides an output of range 0 to10 volts.

A signal from the averaging filter and output amplifier 104 is appliedto the alarm logic and controller 106 which is set to trigger when a setlevel is exceeded. There is also an arrangement for setting the alarmlogic and controller 106 to trigger when the applied signal falls belowa set threshold.

Referring to FIG. 3 of the accompanying drawings, the principalcomponents of the input amplifier are a differential-input operationalamplifier 20, a third capacitor-resistor network consisting of acapacitor 21 in parallel with a resistor 22, a fourth capacitor-resistornetwork consisting of a series-connected capacitor 23 and resistor 24 inparallel with a resistor 25, a fifth capacitor-resistor networkconsisting of a series-connected capacitor 26 and resistor 27 inparallel with a resistor 28, two transmission gate switches 29 and 30,and resistors 19, 31, 32, 33 and 34.

The resistor 19 is connected in series with the inverting input terminalof the operational amplifier 20. The third, fourth and fifthcapacitor-resistor networks are connected in series with one another inthat order between the inverting input terminal of the operationalamplifier 20 and its output terminal. The resistors 31 and 32 areconnected in parallel with each other and connect the junction of thefourth and fifth capacitor-resistor networks to a first terminal of thetransmission gate switch 29. A second terminal of the transmission gateswitch 29 is connected to the system ground and a third terminal of theswitch is connected to the output terminal of the operational amplifier20. The resistors 33 and 34 are connected in parallel with each otherand connect the junction of the third and fourth capacitor-resistornetworks to a first terminal of the transmission gate switch 30. Asecond terminal of the transmission gate switch 30 is connected to thesystem ground and a third terminal of the switch is connected to thejunction between the fourth and fifth capacitor-resistor networks.

The transmission gate switch 30 is controlled by a logic signal CTRL Y.When the signal CTRL Y is high, the first terminal of the transmissiongate switch 30 is connected to its second terminal and when the signalCTRL Y is low, the first terminal of the switch is connected to itsthird terminal. The transmission gate switch 30, therefore is operableeither to connect the resistors 33 and 34 to the system ground (CTRL Yhigh) or to connect those resistors to the junction between the fourthand fifth capacitor-resistor networks (CTRL Y low), that is, in parallelwith the resistor 25.

The transmission gate switch 29 is controlled by a logic signal CTRL X.When the signal CTRL X is high, the first terminal of the transmissiongate switch 29 is connected to its second terminal and, when the signalCTRL X is low, the first terminal of the switch is connected to itsthird terminal. The transmission gate switch 29, therefore, is operableto connect the resistors 31 and 32 to the system ground (CTRL X high) orto connect those resistors to the output terminal of the operationalamplifier 20.

The transmission gate switches 29 and 30 set the overall gain oftransimpedance amplifier 20 to one of four possible values as follows:

CTRL X low, CTRL Y low: gain is 0.1 mV/pA

CTRL X low, CTRL Y high: gain is 0.4 mV/pA

CTRL X high, CTRL Y low: gain is 10 mV/pA

CTRL X high, CTRL Y high: gain is 40 mV/pA

The logic signals CTRL X and CTRL Y are generated by the gain-changelogic circuit 5 shown in FIG. 1.

Referring to FIG. 4 of the accompanying drawings, the principalcomponents of the coupling network and second stage amplifier areresistors 301 and 303 to 308, a capacitor 302, a differential-inputoperational amplifier 309, a transmission gate switch 310, and resistors311, 312 and 313.

The capacitor 302 is connected in series with the non-inverting inputterminal of the operational amplifier 309, the resistor 301 is connectedin series with the capacitor 302. The resistors 303 and 308 shunt thenon-inverting input terminal of the operational amplifier 309 to aground terminal. The resistors 304 and 307 are not normally fitted; oneor other may be fitted as required to increase the range of thepotentiometer 306. The potentiometer 306 and the resistors 305 and 308serve to adjust and possibly remove the input offset voltage of thesecond-stage amplifier 309.

The capacitor 302 and the resistor 303 serve as the dominant elements ofthe coupling network connected to the non-inverting input terminal ofthe operational amplifier 309. The capacitor 302 serves to block d.c.signals but is large enough to permit signals of 0.15 Hz and above topass. The capacitor 302 may have a value of the order of 0.5 μF. Thecapacitor 302 and the resistor 303, the value of which is of the orderof 2 MΩ contribute towards setting the lower break frequency for theamplifier system at about 0.15 Hz.

The resistors 311, 312 and 313 form a network which defines the voltagegain of the second-stage amplifier 309. The resistor 313 is connectedbetween the inverting input and the output of the amplifier 309. Theresistor 311 is connected between the inverting input of the amplifier309 and the system ground. The resistor 312 is connected between thecommon terminal of the transmission gate switch 310 and the invertinginput of the amplifier 309 so that when the signal CTRL X is high, theresistor 312 is connected to the system ground, and when the signal CTRLX is low, the resistor 312 is connected to the output of the amplifier309. When the signal CTRL X is high, the gain of amplifier 309 is five,and when the signal CTRL X is low, the gain of the amplifier is two.

Since the signal CTRL X controls the transmission gate switches 29 and310, the gains of the input amplifier 20 and the second-stage amplifier309 are switched together. That provides selectable overall gains fromthe input of amplifier 20 to the output of the amplifier 309 of 0.2mV/pA, 2 mV/pA, 20 mV/pA and 200 mV/pA.

We have found that by processing the signal as described above an outputthat provides an accurate indication of particle mass flow rate (forconstant particle size, material and velocity) can be obtained and,especially, we have found that in practice a much more accurateindication is obtained by processing the alternating component of thesignal rather than the absolute level of the signal. The difference inthe two techniques is shown qualitatively in FIGS. 5a to 5d. FIG. 5ashows one form of current signal that we have found can be produced froma probe when the average mass flow rate of particles is constant. Itwill be noted that the signal starts with a current which has a larged.c. component of amplitude a and a relatively small a.c. component ofamplitude b. With the passage of time, however, variables in the ductchange: perhaps the humidity of the gas flow changes, perhaps there is abuild-up of charged particles on the probe or perhaps a static charge onthe particles upstream of the probe changes; as a result, the d.c.component of the current reduces or even, as shown in FIG. 5a, changesin polarity, but the smaller a.c. component remains relatively constant.FIG. 5b shows the output signal that is obtained from monitoring theabsolute value of the current, including the alternating component, andthen rectifying the output, that being the procedure adopted in thepreferred embodiment of WO 86/02454. FIG. 5c shows the output signalthat is obtained from monitoring the absolute value of the current,excluding the alternating component, and then rectifying the output.FIG. 5d shows the output signal that is obtained from monitoring theamplitude of the alternating component of the current in accordance withthe invention. It will readily be seen that the output signals of FIGS.5b and 5c suggest that the mass flow rate of particles is changing,whereas the output signal of FIG. 5d correctly indicates a constant massflow rate of particles. The variation in the d.c. component of thecurrent is caused by variations in variables in the duct other thanthose related to the particle mass flow rate, particle size or particlevelocity.

FIG. 6 shows the results, in the form of current in picoamperes plottedagainst time in minutes, of an experiment carried out to compare theresults of monitoring the a.c. and d.c. components of current in thesignal from the probe. In the experiment coal dust particles ofsubstantially constant size were added at a substantially constant rate,of the order of 500 mg m⁻³ to an air flow of 25 m s⁻¹. In FIG. 6 plot Ashows the a.c. component of the probe current and plot B shows the d.c.component. It can be seen that although the particle flow rate issubstantially constant, the d.c. signal alters radically, even changingpolarity whereas the a.c. signal gives a much more consistent output.

I claim:
 1. A method for detecting particles flowing in a gas flow alonga stack and emitted through the stack in which a probe is positioned sothat it projects into the flow of particles in the stack and is chargedtriboelectrically by particles in the flow and the quantities ofelectrical charges transferred to the probe are evaluated to provide anindication of the particle flow in the gas flow, wherein, in order toreduce the effect of variations in gas flow related variables other thanthose relating to particle flow, an alternating component in the signalcaused by the triboelectrical charging of the probe is monitored, thealternating component of the signal from the probe is filtered toexclude high frequency components of the signal and the magnitude of theresidual alternating component is itself used to give an indication ofthe particle flow through the stack.
 2. A method according to claim 1 inwhich the alternating component of the signal from the probe is filteredto limit the frequency to below about 2 Hz.
 3. A method according toclaim 2 in which the alternating component of the signal from the probefiltered to limit the frequency to below about 1.5 Hz.
 4. A methodaccording to claim 1 in which the alternating component of the signalfrom the probe filtered to limit the frequency to above about 0.1 Hz. 5.A method according to claim 4 in which the alternating component of thesignal from the probe is filtered to limit the frequency to above about0.15 Hz.
 6. A method according to claim 1 in which the alternatingcomponent of the signal from the probe is amplified in a plurality ofstages.
 7. A method according to claim 6 in which frequencies belowabout 0.15 Hz are attenuated at the first stage of amplification.
 8. Anapparatus for detecting particles in a gas flow along a stack andemitted through the stack, the apparatus including a probe forinstallation in the gas flow, an electric circuit for generating anelectric signal from electrical charges transferred to the probe as aresult of triboelectric charging thereof by particles in the gas flow,and evaluating means for providing an output signal in dependence uponthe electric signal generated in the circuit, wherein the apparatus isused to reduce the effect of variations in gas flow related variablesother than those relating to particle flow, the electric signal that isgenerated by the circuit has an alternating component and the evaluatingmeans is arranged to provide an output in dependence upon the magnitudeof the alternating component of the electric signal after filtering andcomprises filter means to filter the alternating component of the signalto exclude high frequency components of the signal.